Flamelet modeling of forced ignition and flame propagation in hydrogen-air mixtures

Abstract

Modeling hydrogen-air flames is challenging for several reasons. Due to its high diffusivity and reactivity, the combustion properties of hydrogen are substantially different from conventional hydrocarbon fuels. Besides differential diffusion and flame stretch, which play an important role in hydrogen combustion, the prediction of ignition events is also relevant for the safety and control of combustors operating with hydrogen. To address these effects with a manifold-based method, a novel flamelet progress variable (FPV) model is presented which consists of a tabulated manifold and a coupling strategy. In contrast to prior work, the manifold is coupled to the CFD simulation by transporting major species and enthalpy instead of transporting the flamelet control variables only. A closure approach is developed to employ a mixture-averaged diffusion model accounting for exact diffusion coefficients. Furthermore, a novel tabulation strategy is presented which is based on a recently published composition space model (CSM). The CSM is used for computing and tabulating flamelets which are consistently blended with constant-pressure homogeneous ignition solutions at elevated temperatures. The FPV model is validated for planar and curved hydrogen-air premixed flames across a range of equivalence ratios (0.7≤ϕ≤1.4). The latter are ignited by an energy deposition followed by an outward propagation as cylindrical or spherical expanding flames. It is shown that the FPV model accurately recovers the characteristics of the forced ignition process in both quiescent mixtures and a counterflow configuration, as well as the flame structures and propagation speed of the outwardly propagating hydrogen-air flames.